(Background)
The strategy for achieving higher conversion efficiencies in tandem solar
cells is to increase the number of subcells while the current matching
condition is fulfilled. This means that the difference in bandgap among
the subcells should gets smaller. Simultaneously the optimum bandgap of
the top cell should increase and is estimated to be larger than those achievable
in group-III phosphides or arsenides. Thank to their large bandgaps, group-III
nitrides can be a possible candidate for the materials of top cells in
such an ultimate situation. Methods for good optical and electrical contacts
between nitide top cells and subcells made of other materials must be established
for realizing high-efficiency tandem cells in this scheme. In this work,
possibility of nitride-based cells as the top cell were explored by fabricating
nitride/phosphide and nitride/Si tandem cells using SAB and characterizing
their properties.

(Result)
We prepared nitride-based cells by growing p-on-n cell structures on n-doped
GaN substrates and employing device process sequences. The backsides of
GaN substrates were polished so that its averaged roughness was improved
<1 nm (See below left). Thank to the improved roughness, the backsides
of GaN substrates were successfully bonded to InGaP- and Si-based bottom
cells so that the nitride-on-Si and nitride-on-phosphide hybrid tandem
cell structures were obtained. The schematic cross section of nitride-on-Si
hybrid tandem cells is shown below right. The performances of tandem cells,
top cells, and separately-made bottom cells were measured using an in-house
solar simulator.

The I-V characteristics of nitride-on-(111)-Si tandem cells as well as nitride
top cells are shown in the bottom left figure. The characteristics of (111)-Si
cells are shown in the bottom right. Their open-circuit voltage (VOC) is 1.64 V (tandem cells), 1.12 V (top cells), and 0.56 V (bottom cells).
The short-circuit current (JSC) of tandem cells, top cells, and bottom cells is 0.22, 0.21, and 24.0
mA/cm2, respectively.

Results of characterization of nitride-on-phosphide tandem cells as well
as nitride top cells are shown in the bottom left figure. The characteristics
of phosphide cells are shown in the bottom right. Their VOC is 1.84 V (tandem cells), 0.94 V (top cells), and 1.35 V (bottom cells).
JSC of tandem cells, top cells, and bottom cells is 0.19, 0.17, and 7.42 mA/cm2, respectively.

JSC of tandem cells is close to that of top cells, which means that the performance
of tandem cells is limited by that of top cells. More importantly, in both
nitride-on-Si and nitride-on-phosphide structures, VOC is enhanced in comparison with those of respective subcells. Especially
VOC of nitride-on-Si tandem cells is approximately equal to the sum of those
of nitride top cells and Si bottom cells. This result implies that the
two subcells are electrically well-connected to each other. (Note) This work was jointly performed with Univ. of Fukui and NTT Photonics
Laboratories.